1. Field of the Invention
The present invention relates to an optical projection system to be used for an exposure of a selected pattern on a work by irradiating a projection light obtained by passing an illumination light through a mask.
2. Description of Relevant Art
An apparatus to be used for providing patterns of an electronic circuit of a circuit board, a liquid crystal panel, and a LCD color filter has been widely spread. As an example of these kinds of apparatus, a lithography apparatus that performs the exposure by irradiating ultraviolet light of predetermined wave length can be cited.
Various kinds of optical projection system or catadioptric optical system to be adopted in a lithography apparatus have been brought to market.
A catadioptric optical system 50 disclosed in Japanese Patent publication H10-509561 is one example of the optical projection system of the lithography apparatus.
This catadioptric optical system 50, as shown in FIG. 6, includes a reticle (mask) 51, plates 52 and 58, prisms 53 and 57, a plano-convex lens 54, a meniscus lens 55, and a reflection mirror 56.
In this catadioptric optical system 50, the plate 52 is positioned between the reticle (mask) 51 and the prism 53, and the plate 58 is similarly positioned between a work 59 and the prism 57.
Each of plates 52 and 58 are allowed to bend in lengthwise and crosswise direction. Therefore, the magnification of an image (pattern) to be focused on the work 59 that is formed by a projection light passed through respective plates 52 and 58 can be controlled by changing the curvature of respective plates 52 and 58.
Referring to FIG. 6, the illumination light passed through the mask 51 having a selected pattern thereon passes through the plate 52, and is then directed to the plano-convex lens 54 by the prism 53. The illumination light reflected by the prism 53 passes through the plano-convex lens 54 and the meniscus lens 55, and then strikes the reflection mirror 56. When the illumination light strikes the reflection mirror 56, the illumination light is reversed and is directed to the prism 57. Then, the illumination light reflected by the reflection mirror 56 passes through the meniscus lens 55 and the plano-convex lens 54, and is irradiated on the work 59 after reflected by the reflection mirror 56.
Thereby, the image of the selected pattern on the mask 51 is focused on the work 59. In this occasion, since a magnification of the pattern to be focused on the mask 51 can be varied in compliance with the curvature of the plate 52, the apparatus adopting this catadioptric optical system 50 is allowed to handle various kinds of works differing in size.
A catadioptric optical system 60 disclosed in Japanese unexamined patent publication H8-179217 is the other example of an optical projection system or catadioptric optical system to be adopted in a lithography apparatus.
The catadioptric optical system 60, as shown in FIG. 7A and FIG. 7B, includes a roof prism 61, a right-angle prism 62, a plano-convex lens 63, a concavo-convex lens 64, a meniscus lens 65, a reflection mirror 66, and a magnification compensator 70.
Referring to FIG. 7A, the illumination light irradiated from a light source is directed to the reflection mirror 66 by the roof prism 61, and strikes the reflection mirror 66 after passing through the plano-convex lens 63, the concavo-convex lens 64 adjoined to this plano-convex lens 63, and the meniscus lens 65. When the illumination light strikes the reflection mirror 66, the illumination light is reversed and is directed to the right-angle prism 62. The illumination light reflected by the reflection mirror 66 passes through the meniscus lens 65, the concavo-convex lens 64, and the plano-convex lens 63, and then strikes the right-angle prism 62. Thus, the illumination light reflected by the right-angle prism 62 is irradiated on the work via the magnification compensator 70.
The magnification compensator 70 of the catadioptric optical system 60 includes a plano-concave lens 71, a plano-convex lens 72, and an actuator 73. In this magnification compensator 70, the plano-convex lens 72 is spaced a predetermined distance from the plano-concave lens 71, and this distance is adjusted by the actuator 73.
In case of catadioptric optical system 50, the magnification of the image of the selected pattern to be focused on the work is adjusted by bending mechanically respective plates 52 and 58. Thus, a fatigue of plates 52 and 58 may be caused due to a bending stress. If such fatigue arises, the accurate adjustment of the magnification of the image may be disturbed.
In case of catadioptric optical system 60, the magnification of the image or the selected pattern to be focused on the work is adjusted by changing the distance between the plano-concave lens 71 and the plano-convex lens 72. Thus, the control of the occurrence of the distortion may be difficult.
In the conventional reflection compensation system, since the cemented lens is adopted, this cemented lens has an advantage with regard to the chromatic aberrations. But if ultraviolet lay is adopted as the illumination light (projection light), the disadvantage due to the adoption of the cemented lens has occurred. That is, the ideal performance required for an exposure of the selected pattern on a work by irradiating a projection light cannot be fully obtained due to the degradation of the focusing of the image resulting from the cemented lens. In other words, the ideal performance is not fully obtained due to the variation of aberration, which is the deviation from the ideal focus.
Therefore, the optical projection system to be adopted in the lithography apparatus that can control the magnification without causing the degradation of the focusing of the image and can control the occurrence of the distortion has been required. Additionally, the optical projection system to be adopted in the lithography apparatus that can control the magnification with accuracy, even if the light having low wavelength than ultraviolet ray is adopted as the illumination light, has been required.